We propose to utilize the congenitally athymic (nude) mouse and normal littermates in studies designed to elucidate the nature and function of the intestinal immune mechanisms involved in the control of bacteria and parasitic animals of the gut. Experimental maneuvers designed to specifically dissect the immune response involves the use of heterologous anti-mouse immunoglobulin (Ig) serum which, when injected into nude mice, yields mice totally deficient in both cell-mediated and humoral immunity. Similarly, the treatment of normal littermate mice with anti-micron, yields mice deficient in humoral but not cell-mediated immunity. Thus, using animals whose immune responses are dissected in various ways, we hope to more precisely define the role of cellular and humoral factors in resistance to normal and pathogenic gut flora.